1. Field of the Invention.
The present invention relates to a tooling fixture for precision machining operations that uses a cassette and cassette holder for clamping and for positively placing small precision parts in a reference position for grinding or machining precision parts that are to be ground or machined.
2. Description of the Prior Art.
Precision machining operations on very brittle, and thin parts such as ferrite read/write cores for magnetic heads in computer applications, have been a substantial problem because the parts are extremely small, very brittle, and have to have the core ground down to form a narrow flange on one side of the core. Compounding the problem is the fact that the cores have holes in them, and the hole actually extends into the grinding path that is needed. At the present time, the parts are clamped with rubber-faced clamps, and held against a reference surface, which is ground on a large tooling fixture. The parts are only in the range of 0.060 inches square, and the portion of the part that is to be ground has to protrude out of the clamp so that the grinding wheel can engage it. That leaves a very small surface area on the part for clamping. The rubber on rubber-faced jaws tends to extrude out around the outer edges and also into the openings in the parts. Because the available clamping surface area is quite small, attempting to hold the parts tightly when the parts are being clamped results in high localized loads and excessive breakage, as well as the difficulty in removing the parts after they have been ground.
The edges of the parts opposite from the ground ends are supported on a narrow shoulder or ledge surface because close tolerances are needed, and since the parts have very sharp corners, the corner where the ledge surface joins the reference surface against which the parts are clamped has to be relieved, with a relief groove. That leaves the edge portion of the part resting on the support ledge unsupported on a side opposite the clamping side. The clamping forces of rubber, which again tends to extrude, will place sufficient load on this unsupported edge portion so many parts will crack when they are loaded against the reference surface, because they are unsupported where the relief groove at the corner junction with the support shoulder is formed.
The ferrite parts also have irregularities on the side surfaces which are clamped and such irregularities also can cause cracking when the parts are clamped.
All of these problems result in a high breakage factor, and difficulty in loading and unloading the parts. A sensor is used for controlling tool table movement along the axis of the grinding wheel and the sensor rides on an exposed portion of the reference surface. During grinding operations, the table is moved in direction parallel to the reference surface. A number of parts can thus be ground at once, and the sensor sliding along the reference surface will tend to wear grooves or irregularities in the reference surface. When the wear exceeds tolerances the entire fixture, in the prior art devices, has to be reground or replaced, and this is time consuming and quite expensive. Downtime is a problem because the fixtures are so expensive that having extra fixtures is avoided.
The present invention provides for a cassette designed to alleviate the excessive breakage problems and makes the parts easy to load and unload, as well as providing a replaceable reference bar that provides not only a reference clamping surface but also a reference surface for sensor travel.